1. Field of the Invention
The present invention relates to the registration of multiple medical imaging modalities in support of real time optimization of medical procedures. In particular, the invention relates to the registration of ultrasound and C-arm fluoroscopy imagery for these purposes.
Adenocarcinoma of the prostate is the most commonly diagnosed cancer in the U.S. male population. During the last half decade, there have been approximately 200,000 new cases of prostate cancer diagnosed each year, which is comparable to breast cancer diagnosis, and there is no evidence that this number would significantly decrease in the foreseeable future. For several decades, the definitive treatment of low-risk prostate cancer was radical prostatectomy or external beam radiation therapy (EBRT). During the 90's the technique of transrectal ultrasound (TRUS) guided transperineal low dose-rate brachytherapy became a well-proven treatment alternative, comparable to surgery and EBRT, has demonstrated excellent local control rates, and most recent data reports excellent long-term (10-12 year) disease-free survival rates equivalent to radical prostatectomy and external beam radiotherapy. Moreover, studies indicate that there are no unexpected urethral or rectal complications or side effects with low dose-rate brachytherapy.
However, the rate of rectal and urethral complications associated with low dose-rate brachytherapy is still high. The underlying reason for this has predominantly been the lack of adequate visualization and control of the implant process, which leads to improper placement of the implanted radiation source and inaccurate delivery of the sources of radiation dosage. As one skilled in the art will readily appreciate, radioisotopes are used for non-invasive visualization and control processes such as low dose-rate brachytherapy. There have been advances in the ability to use different isotopes to achieve better dose distributions. However, even the best isotope is useless without adequate visualization, localization, and subsequent control of the implanted radiation source. The ability to intraoperatively localize implanted radiation sources (hereinafter “seeds”) relative to the prostate is key to enabling dynamic dose calculation during the procedure. The solution of this visualization and control problem would reduce the probability of inaccurately placed implants, thus presenting an opportunity for improved outcomes.
TRUS imaging generally provides satisfactory differentiation of relevant soft issue, but implanted brachytherapy seeds cannot be clearly identified in the TRUS images. Advancements in ultrasound equipment technology are expected to facilitate identification and localization of the seeds in the future, but such equipment will not be available for practitioners in the foreseeable future. On the other hand, currently sixty percent or more of the practitioners use intra-operative C-arm fluoroscopy for purposes of visualization, localization, and control. Although C-arm fluoroscopy, which employs transluminal X-Ray imagery, can accurately localize seeds, it does not differentiate soft tissue. Hence, there is a strong demand for using C-arm fluoroscopy together with a TRUS-guided delivery system.
2. Discussion of the Related Art
Problems and limitations inherent in the related art may be properly illustrated by discussing a particular combination of imaging modalities using C-arm X-ray fluoroscopy and transrectal ultrasound (TRUS). C-arm X-ray fluoroscopy may be the most widely used intra-operative imaging modality in general surgery, and approximately 60+% of the prostate brachytherapy practitioners use it for qualitative implant analysis in the operating room in non-computational qualitative manner. While C-arm fluoroscopy has been used intra-operatively, it has not been utilized in quantitative intra-operative analysis. C-arm has been used as a solo guidance modality. However, since TRUS emerged as a primary image guidance modality, C-arm fluoroscopic x-ray imaging has become a secondary tool for gross visual observation. Very few attempts have been made to relate fluoroscopic images to soft tissue anatomy, and with little success. These attempts have generally used thin metal wire inside a Foley catheter to visualize the prostatic urethra fluoroscopically in anterior-posterior and lateral projections. In other approaches, gold marker seeds have been implanted into the prostate, and the relative positions of the needles and marker seeds have been observed in fluoroscopy.
X-ray radiography has been used extensively for post-implant brachytherapy evaluation using multi-view X-ray to recover seed locations post implant and determine gross dosimetry. Here the fundamental problem is matching a large number of seeds with their projections in multiple X-ray images when some seeds obscure each other and solid objects also can get in the way. Automated methods have been explored, but they also assume conditions that most likely cannot be met with a C-arm fluoroscope, particularly in a realistic intra-operative scenario. Such assumptions include no extrinsic object in the field, optimal beam energy, arbitrary number and orientation of X-ray shots, or unlimited processing time and computational resources—none of which is realistic in real-time image-guided surgery. In conclusion, “off-the-shelf” post-implant seed matching and reconstruction techniques of the related art cannot be expected to work in the operating room. Many approaches have met with great difficulty due to the inability to accurately determine the imaging angles relative to the prostate for reconstruction of the multiple projection images. This not only reduces accuracy, but makes the process too lengthy to be used intraoperatively.
The use of implanted needles as fiducial markers for registration of biplane TRUS data has been explored, but several key problems have been left unsolved. First, the use of implanted needles as fiducials may not be practical, because most practitioners implant only one needle at a time and they do not use stabilizing needles. Second nearly parallel transperineal needles, when used as fiducials, encode weakly in the apex-base direction, with little spatial resolution. Third, because X-ray and TRUS imaging are not simultaneous, it is imperative that the fiducials do not move relative to the prostate until both imaging sessions are complete. For example, during TRUS scanning, the prostate deforms, dislocates, and the needles dislocate relative to the prostate. Fourth, previous attempts at using implanted needles as fiducial markers did not account for the need to pre-operatively calibrate the C-arm fluoroscope, including removing image distortion and some form of intra-operative tracking to know where the multiple X-ray shots are coming with respect to one another.
Accordingly, TRUS guided transperineal low dose-rate brachytherapy has emerged as one of the preferred treatments for low-risk prostate cancer. While ultrasound is generally an excellent tool for guiding the implant needles with respect to prostate anatomy, it cannot reliably show the location of radioactive seeds after they are released in the prostate. Intraoperative C-arm fluoroscopy does adequately show the location of implanted seeds, but it cannot differentiate soft tissue; thus, it cannot be used for visualizing prostate anatomy.
Accordingly, what is needed is intra-operative fusion of these two complementary modalities, which offers significant clinical benefit by allowing for real-time optimization of the brachytherapy implant as the procedure progresses in the operating room.
The problems discussed are not limited to TRUS guided transperineal low dose-rate brachytherapy discussed above, but apply to procedures requiring accurate placement of objects such as surgical tools or implants within a patient. The problems and limitations apply to any procedure in which two imaging modalities are used, and in which there is a need to fuse or merge the products of the two imaging modalities into a single image space.